A large body of evidence suggests that skeletal interstitial fluid flow (IFF) mediates bone remodeling in response to mechanical loading. Our long-term goal is to develop treatments for bone loss based on bone cell mechanotransduction of IFF. A crucial step towards this goal is to understand how the responses of bone cells exposed to IFF result in bone remodeling. Recently, Tatsumi and colleagues generated a transgenic mouse model with inducible osteocyte ablation. Interestingly, although these mice were resistant to bone loss upon hindlimb suspension, mechanotransduction in these mice upon reloading was normal. This gives rise to the intriguing possibility that while osteocytes regulate IFF-mediated bone loss, increases in IFF during reloading may induce bone formation by direct stimulation of other types of bone cells. Our central hypothesis is that IFF mediates the bone resorption and formation that occur during unloading and reloading by two distinct mechanisms. During unloading, lack of IFF results in osteocyte- mediated bone resorption. During reloading, increased IFF induces bone formation by direct stimulation of osteoblasts or their progenitors. Recently, our lab has developed a novel microfluidic device for generating dynamic IFF in the femurs of hindlimb suspended mice. The device will be used to determine the role of osteocytes in mediating flow-induced bone remodeling within a native tissue environment. Specifically, we will first induce IFF in hindlimb suspended mice with and without ablated osteocytes to determine the capacity of IFF to inhibit osteoclastic activity upon disuse, and the role of osteocytes in mediating this process (aim 1). Next, by imposing a period of hindlimb suspension pre-osteocyte ablation, we will determine the capacity of IFF to stimulate recovery of disuse-induced bone loss in osteocyte-deficient mice (aim 2). Our findings will reveal fundamental insight into the cellular mechanisms involved in IFF-regulated bone remodeling and will represent a considerable advancement towards development of therapies for bone loss based on mechanotransduction of IFF. Relevance to Public Health: Osteoporosis is a major health risk for 28 million Americans. This study will reveal fundamental insight into how different cells within bone coordinate their responses to drive bone remodeling in response to interstitial fluid flow. Our findings will represent a considerable advancement towards development of therapies for bone loss based on the capacity of bone cells to sense and respond to fluid flow.